Multi-port converter structure for DC/DC power conversion

ABSTRACT

A module for interconnecting a pair of DC sources or a pair of DC loads into a DC bus includes: a first port for each source or load; a switching cell for each first port, each cell having a pair of terminals and a switching node; a second port operatively connected to the DC bus and having a pair of terminals, one of the pair of terminals of the second port being connected to one of the terminals of one of the cells and the other of the pair of terminals of the second port being connected to one of the terminals of the other of the cells; and a filter inductor connected between the switching nodes of the cells. Systems including the module and methods utilizing the system are also disclosed.

CROSS-REFERENCE TO CO-PENDING APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/789,466 filed on Oct. 20, 2017, which is a continuation-in-part ofU.S. patent application Ser. No. 15/308,566 (now U.S. Pat. No.10,291,123), which is a national phase entry of PCT patent applicationPCT/CA2015/050361, filed on Apr. 30, 2015, which claims priority to U.S.Provisional Patent Application 61/987,746, filed May 2, 2014, Thisapplication also claims all benefit including priority to U.S.Provisional Patent Application No. 62/411,168, filed Oct. 21, 2016, andentitled “MULTI-PORT CONVERTER STRUCTURE FOR DC/DC POWER CONVERSION”.

FIELD

This invention relates to the field of power converters for dc systems.

BACKGROUND

A two-quadrant buck converter, also known as a synchronous buckconverter, is a type of basic switch-mode dc/dc converter that is usedto regulate voltage and provide efficient dc power transfer in energysystems. The traditional two-quadrant buck converter cell, shown in FIG.20 (prior art), comprises a pair of complimentary power switches andinput capacitor. An output L-C low-pass filter is employed when a smallhigh frequency ripple for the output voltage is required. For steadystate operation, switch S₁ is turned “on” (i.e. switch S₁ is closed) andS₂ is turned “off” (i.e. switch S₁ is opened) during time intervalD·T_(s). The converter duty cycle D, which ranges from 0 to 100%,represents the percentage time when switch S₁ is on (and thus when S₂ isoff) during switching period T_(s). During each time interval D·T_(s),voltage v_(x) at node x becomes equal to input voltage V_(in) as shownin FIG. 20. Voltage v_(x) becomes zero when switch S₁ is turned off (andthus when S₂ is turned on) for the remainder of the switching period,due to the complimentary switching action of S₁ and S₂. Based on thisdiscussion, the voltage v_(x) can be viewed as having an average valueof D·V_(in) with a set of high frequency switching harmonics. The L-Clow-pass filter is designed such that it attenuates the high frequencyswitching harmonics of v_(x) and allows the output voltage V_(out) to beequal to the average value D·V_(in). Assuming the output voltage isexternally regulated, the inductor current I_(L) can be made to take oneither a positive or negative average value through adjustment of theconverter duty cycle, thus enabling bidirectional energy transferbetween input and output terminals. Therefore, voltage regulation andbidirectional energy transfer can be achieved easily by suitable controlof the duty cycle D in a two-quadrant buck converter.

It should be understood that a unidirectional variant of thebidirectional buck converter in FIG. 20 can be realized, by, forexample, replacing switch S2 with a diode. The unidirectional buckconverter can be employed for applications where only input to outputpower transfer capability is needed. Multiple two-quadrant buckconverter cells with associated filters can also be connected in seriesto form “classical cascaded buck converters”. FIG. 21 (i.e. prior art)shows a classical cascaded buck converter comprised of three individualdc/dc buck converter cells, each with associated output filtering,connected in series. The topology shown has three input ports and oneoutput port. Each of the input ports and the output port consists of twoterminals as shown. Observe each input port has an assigned referenceterminal with its voltage defined relative to ground, i.e. v_(n1),v_(n2) and v_(n3). By chosen convention the reference terminals areselected such that they correspond to the bottom connection point ofeach input port capacitor. To limit voltages to ground, a singlereference terminal is typically connected to ground. In FIG. 21, thereference terminal selected for ground connection is shown by the dottedconnection from v_(n3) to ground. However, it must be stressed thischoice is entirely arbitrary, i.e. any other reference terminal in FIG.21 could have been connected to ground. The classical cascaded buckconverter allows multiple input ports to exchange energy with a commonoutput port, wherein the output voltage can be significantly higher thanindividual input voltages. This flexibility makes the classical cascadedbuck converter suitable for a wide range of applications such asphotovoltaic systems and battery management units.

Present state-of-the-art technology having similar application andfunctionality compared to the classical cascaded buck converter in FIG.21 is the cascaded connection of two-quadrant buck converter cells thatshare a single L-C low-pass filter, shown in FIG. 22. However, withexception of the one ground-connected reference terminal, all otherinput reference terminal voltages for this topology, i.e. v_(n1) andv_(n2), are subject to undesired high frequency switching ripplevoltage. As a result, energy sources that are connected to these inputports, for example, solar panels or batteries, will suffer fromsignificant capacitive current to ground. In contrast, the classicalcascaded buck converter with multiple low-pass L-C filters as shown inFIG. 21 can reduce the high frequency switching ripple voltage on v_(n1)and v_(n2), provided that individual L-C filter elements aresufficiently large. However, this comes at the expense of an overallincrease in both the size and number of energy storage components(inductors and capacitors). The additional components increase the lossand cost of the classical cascaded buck converter.

SUMMARY OF THE INVENTION

It will be evident from the foregoing, and from a review of the detaileddescription that follows, that the multi-port converter topologies fordo/dc power conversion embodied within the apparatus are of significantadvantage, in that, inter alia, they:

-   -   are modular and scalable;    -   can be designed to have an arbitrarily small high frequency        switching voltage ripple magnitude at all input and output        reference terminals;    -   are capable of bidirectional energy exchange between input ports        and output port;    -   are capable of controlling power sharing among the input ports;    -   are capable of allowing multiple inputs to exchange energy with        a common output, wherein the output voltage can be significantly        higher than individual input voltages;    -   have relatively low rating of components; in particular, the net        rating of energy storage components (capacitors and inductors)        are small compared to the classical cascaded buck converter;    -   are highly flexible in that they can be cascaded with modules of        the same topology or cells of differing topology.    -   allow for input and output ports to be re-defined to offer a        wider range of functionality, including options where only 2        ports of the circuit are used.

Other advantages and features associated with the multi-port convertertopology will become evident upon a review of the following detaileddescription and the appended drawings, the latter being brieflydescribed hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Double-input single-output converter module with generalizedpower switches;

FIG. 2 One example of power switches realization using MOSFETs anddiodes for the double-input single-output converter module in FIG. 1;

FIG. 3A One possible gating strategy with corresponding switching statesfor double-input single-output converter module in FIG. 1;

FIG. 3B Equivalent circuit diagram for FIG. 1 corresponding to switchingstate #1: switch S_(1b) and switch S_(2a) turned on;

FIG. 3C Equivalent circuit diagram for FIG. 1 corresponding to switchingstate #2: switch S_(1a) and switch S_(2a) turned on;

FIG. 3D Equivalent circuit diagram for FIG. 1 corresponding to switchingstate #3: switch S_(1a) and switch S_(2b) turned on;

FIG. 3E Equivalent circuit diagram for FIG. 1 corresponding to switchingstate #4: switch S_(1b) and switch S_(2b) turned on;

FIG. 4 Series-stacking ‘k’ double-input single-output converter modulesin FIG. 1 to form a 2k-input single-output cascaded dc/dc converterstructure;

FIG. 5 Series-stacking one double-input single-output converter moduleof FIG. 1 with (k−1) two-quadrant buck converter cells to form a(k+1)-input single-output cascaded dc/dc converter structure;

FIG. 6 (k+1)-input single-output cascaded converter structure wherephysical placements of the module and switching cell plurality areinterchanged relative to FIG. 5;

FIG. 7 One possible cell sorting and selection scheme for cascadedconverter structures in FIG. 5 and FIG. 6 to achieve charge balancing ofinputs across all possible output voltages;

FIG. 8 Simulation model for FIG. 1 implemented in PLECS;

FIG. 9 Simulation results for double-input single-output convertermodule in FIG. 1: power transfer from inputs to output;

FIG. 10 Simulation results for double-input single-output convertermodule in FIG. 1: power transfer from output to inputs, where powertransfer is divided evenly between input ports;

FIG. 11 Simulation results for double-input single-output convertermodule in FIG. 1: power transfer from output to inputs, where powertransfer is divided unevenly between input ports as assigned by theuser;

FIG. 12 Simulation results for prior art comprising four series-cascadedtwo-quadrant buck converter cells with single L-C output filter (i.e. afour input variant of FIG. 22), to demonstrate inability to achieve fulloutput voltage range when only one of the cells utilizes switch-modeoperation;

FIG. 13 Simulation results for a four-input single-output cascadedconverter structure of FIG. 5, to demonstrate ability to achieve fulloutput voltage range when utilizing switch-mode operation for thehigh/low-frequency cell stack;

FIG. 14 Switch gating waveforms for simulation results in FIG. 13, wherehigh-frequency cells are switched at 50 kHz;

FIG. 15 Finer time scale resolution (i.e. zoomed time axis) for a chosensegment of simulated waveforms in FIG. 14, to show contrast betweenhigh-frequency and low-frequency switching times;

FIG. 16 Simulation results for a four-input single-output cascadedconverter structure of FIG. 5 employing battery modules, to illustratemechanism allowing equal charge balancing between input ports such thatthe average battery voltages are depleted at the same rate;

FIG. 17 Switch gating waveforms for simulation results in FIG. 16, wherehigh-frequency cells are switched at 50 kHz and low-frequency cells areswitched at 20 Hz;

FIG. 18 Finer time scale resolution (i.e. zoomed time axis) for a chosensegment of simulated waveforms in FIG. 17, to show contrast betweenhigh-frequency and low-frequency switching times;

FIG. 19 Double-input single-output converter module with functionallysimilar output capacitor configuration

FIG. 20 Prior art: Two-quadrant buck converter with output filtering,depicted along with corresponding operational waveforms;

FIG. 21 Prior art: Classical cascaded buck converter with multiple L-Coutput filters, where three buck converter cells are employed for easeof illustration; and

FIG. 22 Prior art: Cascaded buck converter with a single shared L-Coutput filter, where three buck converter cells are employed for ease ofillustration.

FIG. 23 Single-input single-output application of the converter module,using the functionally similar output capacitor configuration.

FIGS. 24A, 24B, 24C, 24D Equivalent circuit diagrams for FIG. 1corresponding to (a) switching state #1: switch S_(1b) and switch S_(2a)turned on; (b) switching state #2: switch S_(1a) and switch S_(2a)turned on; (c) switching state #3: switch S_(1a) and switch S_(2b)turned on; (d) switch S_(1b) and switch S_(2b) turned on, respectively.

FIG. 25 Proposed general control scheme for the single-inputsingle-output topology.

FIG. 26 Specific example of how to apply proposed for the single-inputsingle-output topology with unidirectional power flow from a solar arraywith maximum peak power tracker.

FIG. 27 Simulation results for a single-input single-outputunidirectional converter structure of FIG. 23 employing solar panels, toillustrate the step up mechanism of the buck-boost operation.

FIG. 28 Simulation results for a single-input single-outputbidirectional converter structure of FIG. 23 employing battery modules,to illustrate the power reversal capability of the proposed converter.

DETAILED DESCRIPTION

The dc/dc converter module shown in FIG. 1 has two first ports, labelledwith voltages “v₁” and “v₂”. and one second port, labelled with voltage“v₃”. In this document, an “input” port refers to a port that connectsto a dc source or load, while an “output” port refers to the port thatis operationally connected to the dc bus. Throughout this document,first ports and “input” ports are used interchangeably, and second portsand “output” ports are similarly interchangeable. Thus, the convertermodule in FIG. 1 is referred to as a double-input single-outputconverter module.

The converter module in FIG. 1 is comprised of two synchronous buckconverter cells with a single filter inductor as shown. A keytopological feature of the double-input single-output converter moduleis the placement of inductor L across the two inner switches S_(1b) andS_(2a) as shown. This configuration results in the inductor beingeffectively “isolated” from the output port terminals. That is, due tothe imposed connection of L across non-matching switches of the two buckconverter cells, the output port terminals may be connected directly toinput port terminals as shown. This natural topological feature is seenas highly advantageous in achieving an arbitrarily small high frequencyswitching ripple magnitude at all input and output reference terminalsin FIG. 1, as it avoids reliance on excessively sized passive filters toachieve this goal.

The converter module in FIG. 1 employs two pairs of complimentaryswitches: 1) S_(1a), S_(1b) and 2) S_(2a), S_(2b). Similar to theconvention illustrated in FIG. 20, duty cycle command D₁ controls thepercentage time that S_(1a) is on (and thus S_(1b) is off) and dutycycle command D₂ controls the percentage time that S_(2a) is on (andthus S_(2b) is off). An interleaved operation of the two pairs ofswitches is possible for this topology, where controlling a relativephase shift, Φ, between D₁ and D₂ can regulate the order in which thefour switches are turned on (over each switching period T_(s)). However,interleaved operation is optional, i.e. interleaved operation of thebuck converter cells in FIG. 1 is not required. The two pairs ofcomplimentary power switches can be implemented using a number ofdifferent switching devices or technologies; one possible example of animplementation using MOSFETs and Diodes is shown in FIG. 2. It should beunderstood that there are many possible implementations of the switchesand energy storage components shown for the double-input single-outputconverter module in FIG. 1. Therefore, such variants are considered asbeing functionally similar to FIG. 1.

Bidirectional energy exchange between the input ports and output port inFIG. 1 is possible. Specifically, power can be transferred either: 1)from the output port to both input ports or 2) from both input ports tothe output port. A salient operational feature of the topology in FIG. 1is that power sharing among the two inputs can be achieved in acontrolled manner, as will be demonstrated in the latter simulationssection. It should be understood that a unidirectional variant of FIG. 2can be easily realized, by, for example, replacing two of the fourMOSFET switches with diodes.

Double-Input Single-Output Converter Module: Theory of Operation

Due to the flexible and scalable nature of the double-inputsingle-output converter module shown in FIG. 1, there are many possiblemethods or strategies in which to operate the converter. Therefore, thesubsequent operational analysis should not be considered to be limiting.For demonstration purposes and ease of understanding, the followingassumptions are imposed to illustrate the key characteristics of thetopology:

-   -   All switching devices and energy storage components (i.e.        inductors and capacitors) are lossless;    -   Dead time for switches is neglected to simplify mathematical        analysis.

Reference is now made to FIG. 1. Switches S_(1a) and S_(1b) arecontrolled by duty cycle command D₁ while switches S_(2a) and S_(2b) arecontrolled by duty cycle command D₂. It is assumed the two pairs ofswitches have the same switching period T_(s), however, this is done forease of understanding as such an assumption is not necessary to obtainthe following results. In general, four possible switching states existfor the converter module in FIG. 1; these four states are illustrated inFIG. 3B, FIG. 3C, FIG. 3D, and FIG. 3E. One possible switching patternthat can generate all four switching states is given in FIG. 3A, where Φis the (per-unitized) relative phase shift between D₁ and D₂ commands.Note in FIG. 3A the order of the switching states is indicated, however,as discussed previously, the order and duration of switching states canvary and thus the assumed diagram in FIG. 3A is not unique. The dutycycle commands for each power switch are defined in FIG. 3A, whereD′₁=1−D₁ and D′₂=1−D₂.

The inductor voltage corresponding to the four possible switching statesof FIG. 1, as illustrated in FIG. 3A, are:State #1:v _(L) =V ₁ +V ₂ −V ₃  (1)State #2:v _(L) =V ₂ −V ₃  (2)State #3:v _(L) =−V ₃.  (3)State #4:v _(L) =V ₁ −V ₃  (4)

The principle of inductor volt-second balance (IVSB) dictates that theaverage inductor voltage over one switching period in steady state iszero. Consequently, the following voltage relationship can be derivedwhen equating the average inductor voltage to zero using IVSB:V ₃=(1−D ₁)V ₁ +D ₂ V ₂.  (5)

The voltage relationship in (5) does not make any assumptions on thevalues of D₁, D₂, Φ, V₁, V₂ or V₃ within their permissible range and isindependent of the switching period, and thus be considered a generaldesign equation for this topology. However, a set of values for D₁, D₂,and Φ can be selected to achieve a minimum inductor ripple current and aminimum capacitor ripple voltage.

Similar to the preceding analysis, a current relationship can also befound using the principle of capacitor charge balance (CCB). Theprinciple of CCB dictates the average capacitor current over oneswitching period in steady state is zero. By equating the averagecapacitor current to zero for each port, the following currentrelationship can be derived

$\begin{matrix}{I_{3} = {\frac{I_{1}}{1 - D_{1}} = \frac{I_{2}}{D_{2}}}} & (6)\end{matrix}$

The polarity of currents in (6) correspond to those assumed in FIG. 1.

Cascaded Topologies Formed by Series-Stacking Multiple Converter Modules

The double-input single-output dc/dc converter module shown in FIG. 1can be extended to form cascaded topologies by stacking multiple modulesin series. FIG. 4 shows one possible example of such a cascaded topologywhere k double-input single-output converter modules are stacked to forma 2k-input single-output structure. Each of the k modules has the sametopology as shown in FIG. 1, but individual modules do not necessarilyneed to employ the same energy storage components, or same realizationof the power switches. The cascaded converter structure shown in FIG. 4enables additional inputs (i.e. more than two) to exchange energy with acommon output, wherein the output voltage can be significantly higherthan individual input voltages.

The stack of k modules in FIG. 4 can be gated (i.e. switched) on/off ata relatively high common switching frequency corresponding tof_(s)=1/T_(s), as is the case with conventional switch-mode converters.Output L-C filter components are designed to attenuate high frequencyripple associated with switch-mode operation. However, in contrast tohigh frequency (i.e. switch-mode) operation, substantially lowerswitching frequencies can also be exploited to operate the majority ofmodules in a “voltage stacking mode”. The term “voltage stacking mode”refers to where select modules, or, select switching converter cellswithin individual modules, are inserted or removed from the cell stackfor extended periods of time. This is inherently different fromconventional switch-mode operation where cells are switched in/out atmuch higher switching frequencies, typically according to some form ofpulse-width modulation. Taking into consideration both switch-modeoperation and voltage stacking mode, values of f_(s) for an individualcell can be anywhere from less than 1 Hz to several hundred kHz. Higherswitching frequencies (i.e. more than several hundred kHz) can also beadopted. There is no requirement the switching frequency of individualcells in FIG. 4 must be equal; in general, each buck converter cell canemploy a different f_(s).

The converter structure in FIG. 4 is only one example of how multipledouble-input single-output converter modules of FIG. 1 can be utilizedto create a cascaded architecture with increased number of inputs.Reference is now made to FIG. 5, where a single converter module fromFIG. 1 is series-stacked with (k−1) two-quadrant buck converter cells(i.e. switching cells) to form a (k+1)-input single-output cascadedstructure. The resulting string of cascaded cells is partitioned into a“high/low-frequency cell stack” and a “low-frequency cell stack”. The“high/low-frequency cell stack” comprises the two switching cells in theconverter module and signifies that these cells are capable ofoperating: 1) both as high-frequency switch-mode converters, 2) both inlow-frequency voltage stacking mode, or 3) in any combination thereof.

The “low-frequency cell stack” in FIG. 5 comprises the remainingswitching cells (i.e. the cells that do not comprise the module) andsignifies that the (k−1) series-cascaded cells operate exclusively in alow-frequency voltage stacking mode. That is, in general, the on-offstates for each pair of complimentary switches in the “low-frequencycell stack” do not change during several successive high-frequencyswitching periods of the neighbouring “high/low-frequency cell stack”.

Switching frequencies employed for the “low-frequency cell stack” can bemany orders of magnitude smaller than the associated switch-modeoperating frequency of the “high/low-frequency cell stack”. It should bestressed that on-off state durations of complimentary switch pairswithin the “low-frequency cell stack” can be made to be arbitrarilylong, i.e. there is no fundamental constraint placed on the maximum ‘on’state duration for each pair of complimentary switches within the“low-frequency cell stack”.

Based on the above discussion, the input port reference terminalvoltages v_(n1,1), v_(n1,2) and v_(n2) to v_(n(k−1)) in FIG. 5 are notsubject to appreciable high-frequency switching ripple voltage. Voltagesv_(n1,1), v_(n1,2) in the high/low-frequency cell stack achieve this byexploiting the structure in FIG. 1, while voltages v_(n2) to v_(n(k−1))achieve this by leveraging a very low frequency operation of thelow-frequency cell stack. Note v_(nk) is connected to ground in FIG. 5and thus does not experience any switching voltage stress.

An advantage of the topology in FIG. 5 over the structures described in,for example, Z. Zheng, K Wang, L Xu and Y Li, “A Hybrid CascadedMultilevel Converter for Battery Energy Management Applied in ElectricVehicles,” IEEE Trans. Power Electronics, vol. 29, no. 7, pp. 3537-3546,July 2014. [“Zheng et al.”] is that it can be designed to havearbitrarily small high-frequency switching ripple magnitude at all inputand output reference terminals, while simultaneously achieving reducedcomponent count, cost, and loss. This is evident when inspecting FIG. 5as only one interface inductor and one output filter capacitor isneeded.

It should be understood the cascaded structure in FIG. 5 is only oneexample of how the basic converter module in FIG. 1 can be arranged withseries-cascaded two-quadrant buck converter cells. Any number ofdouble-input single-output converter modules of FIG. 1 can be utilized(i.e. not only limited to a single module as shown in FIG. 5), as wellas any number of two-quadrant series-cascaded buck converter cells.Other alternatives are possible, such as, for example, the order inwhich individual cells are stacked in series within the resultingstring.

Reference is now made to FIG. 6, where physical placement of thehigh/low-frequency cell stack and low-frequency cell stack areinterchanged relative to FIG. 5.

The topology in FIG. 6 remains a (k+1)-input single-output cascadedconverter structure. Apart from the physical ordering of low-frequencyand high/low-frequency cell stacks within the string, this variantoperates in a manner substantially similar to that of the structure ofFIG. 5.

Full Output Voltage Range Capability

There is a practical restriction on the range of duty cycles a dc/dcconverter can achieve. Namely, a converter is incapable of operatingwith a duty cycle very close to one, or very close to zero. Duty cyclecommands within a small region above zero are in practice set to zero,while duty cycle commands within a small region below one are inpractice set to unity. This practical limitation in the achievable rangeof duty cycles stems from the non-zero turn-on and turn-off timesinherent to any semiconductor based power switching device. In addition,commercial PWM modules are typically incapable of modulating duty ratiosvery close to zero and one. It should be stressed the range ofnon-permissible duty cycles varies depending on many factors such asswitch technology, switch voltage and current ratings, and switchingfrequency. In this work, particularly the simulations section, dutycycle commands less than 0.1 or greater than 0.9 are assumed to beunachievable for switch-mode operating cells. However, this range ofvalues is chosen for illustrative purposes, only, and should be notconsidered as being typical.

The limitation in available range of duty cycles implies there is arange of average output voltages near zero, and a range of averageoutput voltages near V_(in), that a switch-mode dc/dc converter cannotachieve. In Zheng et al., it is explicitly highlighted that the adoptedoperational strategy utilizes only one converter cell for high-frequencyswitch-mode operation. The remaining cascaded buck converter cellsoperate deliberately in a low-frequency voltage stacking mode. Thisimplies only one cell is available to synthesize the deficit portion ofthe reference output voltage not provided by the dedicated low-frequencybuck converter cells. Thus, taking into consideration the practicallimitation for realizable duty cycles as described above, the topologyin Zheng et al. is unable to achieve all possible average outputvoltages. This stems from the fact the single dedicated switch-mode buckconverter cell cannot achieve all duty cycle commands. This deficiencyof prior art will be demonstrated in the simulations section.

In contrast to prior techniques, the cascaded topologies in FIG. 4through FIG. 6 operate in such a manner that achieves all possibleaverage output voltages for the entire cell stack. The effect of theaforementioned duty cycle restriction is alleviated by allowing at leastone double-input single-output converter module, which corresponds to atleast two buck converter cells as illustrated by FIG. 1, to operate in aswitch-mode fashion as needed. The remaining cascaded cells need onlyoperate in low-frequency voltage stacking mode. The key requirement isthat, in order to accommodate all possible deficit portions of thereference output voltage not provided by the low-frequency cell stack,at least one module must be dedicated to switch-mode operation.Additional modules can operate as switch-mode converters, however, thisis not required in order to achieve full output voltage rangecapability. This operational benefit will be demonstrated in thesimulations section. It is important to reiterate the high/low-frequencycell stack is not limited to only one converter module, as shown by FIG.5 and FIG. 6. Multiple modules can be cascaded while realizing the samebenefit of full output voltage range capability.

Charge Balancing of Cells across all Possible Output Voltages

The cascaded converter structures in FIG. 4 through FIG. 6 can be usedfor many applications. In particular these topologies are well suitedfor battery systems. Individual batteries can be connected to the inputports along the entire cell stack, thus enabling their integration witha common dc link for bidirectional energy transfer.

An advantageous operational feature of the proposed cascaded dc/dctopologies for battery systems is that charge balancing of the differentcells, i.e. individual batteries, can be achieved. Moreover, recallingthe preceding discussion on output voltage range capability, thecascaded structures in FIGS. 4-6 can achieve this cell balancing acrossall possible output voltages. Cell balancing is defined in this contextas a means to ensure each battery along the entire cell stack ischarged/discharged at the same average rate. This is seen as a highlybeneficial operating feature for battery energy management systems, asunequal charge depletion/repletion amongst the various batteries can beavoided. Cell balancing is achieved by suitable operation of theindividual cells (ref. FIG. 4) or suitable coordinated operation oflow-frequency and high/low-frequency cell stacks (ref FIGS. 5 and 6). Ofcourse, long term (i.e. average) charge/discharge rates of individualbatteries can deliberately be made unequal, if so desired.

Reference is now made to FIG. 7, which shows a high level diagramillustrating one possible operational strategy to achieve chargebalancing between cells for the cascaded converter structures in FIG. 5and FIG. 6, across all possible output voltages. Here it is assumedthere are a total of N cells in the entire stack, with N−2 cells in thelow-frequency cell stack (i.e. two cells in the module and N−2 cells inthe switching cell plurality). As shown in FIG. 7, there exists a cellvoltage sorting block and a gating logic block. The cell voltage sortingblock acts on a “slow” time scale, corresponding to the switching periodof the low-frequency cell stack, and sorts/orders all input ports basedon their voltage measurements. The gating logic block acts on asubstantially faster time scale, corresponding to the high-frequencyswitching period of the high/low-frequency cell stack, to allow at leastone cell within the high/low-frequency cell stack to operate inswitch-mode. As a result of this combination of slow and fast control,individual cells can be removed and inserted as needed to meet allpossible stack voltage demands (i.e. output port voltage references),while simultaneously ensuring charge balancing between cells. The“output port voltage reference” command in FIG. 7 can be generated byvarious means, such as, for example, by the resulting control action ofan external control logic block. An example application employingbatteries is presented in the simulations section to illustrate thisfunctionality.

It should be stressed that, as previously mentioned, the implementationin FIG. 7 is not unique. There are many other alternate implementationsthat can similarly achieve balancing between cells of the entire stack,by suitable operation of the high/low-frequency and low-frequency cellstacks.

Deployment

The double-input single-output dc/dc converter module shown in FIG. 1,along with the embodiments shown in FIG. 4 through FIG. 6, can be usedto enable bidirectional (or, if desired, unidirectional) energy exchangebetween multiple inputs and a common output, wherein the output voltagecan be significantly higher than individual input voltages. Suchoperation has wide range of application in systems such as, but notlimited to, photovoltaic systems and battery management units. Forexample, individual photovoltaic panels that use centralized ordistributed based maximum power point tracking schemes can be connectedto input ports of a unidirectional variant of the cascaded topologyshown in FIG. 4, thereby allowing maximal energy extraction from eachpanel (at its respective panel voltage) to a local load or external dcnetwork.

Another example of application is to connect battery units to the inputports of the converter topology in FIG. 5 and FIG. 6 to enablecentralized energy management of the battery system. The bidirectionalenergy exchange capability can be leveraged to allow individual batteryunits: 1) to supply energy to a load/source connected at the output or2) to be charged by a dc source connected at the output. Furthermore,individual battery units can be inserted or removed from the batterystack, via appropriate switching action, thus allowing balanced chargingand/or discharging of select battery units for all possible outputvoltages.

Converter Simulations: Introduction

In subsequent sections, the double-input single-output dc/dc convertermodule in FIG. 1 and the (k+1)-input single-output cascaded dc/dcconverter structure in FIG. 5 are simulated using PLECS. Specific casestudy scenarios are simulated to demonstrate key operationalcharacteristics of the converters. In addition, an example case studycomparing performance of the converter module in FIG. 1 and prior art iscarried out.

Simulation Results: Example Case Study Performance Comparison betweenProposed Double-Input Single-Output Converter Module and ClassicalTwo-Input Single-Output Cascaded Buck Converter

The PLECS simulation model of the double-input single-output convertermodule of FIG. 1 is given in FIG. 8.

The nomenclature used in reference to FIG. 8 is summarized in Table 1.

TABLE 1 Nomenclature adopted for PLECS simulations involving Figure 8Quantity Name V_(in) Average voltage of input ports (i.e. averagecomponents of V₁ and V₂) V_(out) Average voltage of output port (i.e.average component of V₃) I_(out) Average current of output port (i.e.average component of I₃) T_(s) Switching period D, D₁, D₂ Duty cycles ΦPhase shift (per-unit) between duty cycles Δv_(out,pp) Peak-to-peakoutput voltage ripple of V₃ Δi_(out,pp) Peak-to-peak output currentripple of I₃ Δv_(c,pp) Equivalent single capacitor peak-to-peak voltageripple Δi_(L,pp) Equivalent single inductor current I_(L) peak-to-peakripple Δv_(n) Voltage ripple of reference terminal voltage v_(n1)

Advantages of the double-input single-output converter module in FIG. 1over the classical two-input single-output cascaded buck converter (i.e.two cascaded buck converter cells with associated filters) can bequantified by comparing them in an application example. Such anapplication example is now considered, which consists of an energymanagement system having two equal battery voltages of V_(in)=60V (i.e.two inputs of 60V nominal rating) with an output voltage requirement ofV_(out)=90V and I_(out)=10A. This corresponds to a power transfer ofapprox. 900 W. Equal energy storage requirements (of inductors andcapacitors) and operating conditions are imposed on the converter modulein FIG. 1 and the classical cascaded buck converter. These imposedconditions allow key performance criteria of the topologies to becompared directly. The performance criteria of interest for this examplecase study are: 1) losses, 2) capacitive current to ground, and 3)harmonic distortion.

The following metric is proposed to evaluate converter performanceMetric

Δi _(L,pp) ·Δv _(n)  (7)

In order to reduce converter losses, capacitive current to ground andtotal harmonic distortion, it is necessary to reduce both Δi_(L,pp) andΔv_(n) simultaneously. In other words, a “better” converter will havesmaller Δi_(L,pp) and Δv_(n) values, and thus a smaller metric scoreconstitutes a preferred converter structure based on the consideredperformance criteria.

FIG. 8 shows the PLECS simulation circuit for the double-inputsingle-output converter module in FIG. 1. In all simulations, theswitching period T_(s) is set equal to 16.67 μs for all switches. Theinductor L is 125 μH, and capacitor C₃ is implemented by connecting two0.926 μF capacitors in series as shown. This is done to ensure fairnessof the comparison, as the simulation model for prior art utilizes twocapacitors of 0.926 μF each and two inductors of 62.5 μH each. Thesizing of energy storage components is chosen based on inductor currentripple and capacitor voltage ripple considerations for the prior art.Based on these parameters, the converter module in FIG. 1 (and also FIG.8) and the classical cascaded buck converter have equal inductive andcapacitive energy storage requirements. Moreover, this also ensuresequal operating conditions for both topologies.

A comparative case study is carried out for the two topologies asdescribed above. Reference is now made to FIG. 9, which presentssimulation results for the double-input single-output converter modulein FIG. 1, where power transfer is approx. 900 W from input ports tooutput port. The depicted waveforms were obtained using the simulationmodel in FIG. 8.

A summary of the case study simulation results comparing the twoconverter topologies is tabulated in Table 2. The classicalseries-cascading of buck converter cells as employed by prior artresults in equal duty cycle commands: D₁=D₂=0.75. However, to achievethe same operating point, the double-input single-output convertermodule in FIG. 1 employs unequal duty cycles: D₁=0.25 and D₂=0.75. Thisis due to the imposed switches arrangement and chosen convention of dutycycle commands, as illustrated in FIG. 3A.

It should be noted that a set of optimal Φ, D₁ and D₂ can be found tominimize Δi_(L,pp) and Δv_(n) for FIG. 1 and prior art separately.

TABLE 2 Summary of case study simulation results comparing performanceof double-input single-output converter module in FIG. 1 (and also FIG.8) and classical cascaded buck converter (prior art) Φ V_(out)(V)I_(out)(A) Δv_(out,pp)(V) Δi_(out,pp)(A) Δv_(C,pp)(V) Δi_(L,pp)(A)Δv_(n)(V) FIG. 1 0 90.2 9.96 8.58 0.954 4.29 2.12 8.53 D₁ = 0.250.25T_(sw) 90.0 9.99 2.23 0.247 1.11 1.02 2.23 D₂ = 0.75 0.50T_(sw) 90.59.97 8.56 0.951 4.28 2.15 8.56 Prior art 0 90.1 9.99 12.6 1.41 6.32 3.176.31 D₁ = 0.75 0.25T_(sw) 90.2 9.96 8.58 0.953 35.4 5.68 35.3 D₂ = 0.750.50T_(sw) 90.0 10.01 2.22 0.246 26.6 2.92 26.4

Table 3 compares the computed values of performance metric (7) for FIG.1 and prior art, based on the case study results summarized in Table 2.It can be seen from Table 3 that the double-input single-outputconverter module in FIG. 1 achieves a better metric score in comparisonto the classical cascaded buck converter, for all values of Φ. Thisimplies FIG. 1 is the preferred topology for the considered performancecriteria.

It is important to recognize the optimal value of Φ for each convertertopology, which is defined in this example as the Φ value correspondingto the lowest metric score in Table 3, is 2.27 for FIG. 1 and 20.03 forprior art. For these optimal conditions, the double-input single-outputdc/dc converter module has a far superior performance (as metric scoreis approx. 10 times lower) and therefore outperforms the classicalcascaded buck converter.

TABLE 3 Summary of key simulation results for converter module andclassical cascaded buck converter (prior art) Φ Figure 1 Prior artMetric 0 18.2  20.03 0.25 T_(sw)  2.27 201.07  0.50 T_(sw) 18.4  77.67

It should be stressed that a case study comparison between thedouble-input single-output converter module in FIG. 1 and cascaded buckconverter with single L-C output filter (i.e. Zheng et al.) is notcarried out, due to the very large high frequency switching voltageripple that naturally occurs with the latter. In this case, use of Zhenget al. would result in a Δv_(n) of 60 V, which implies computed valuesof metric (7) that far exceed those summarized in Table 3.

Simulation Results: Power Transfer from Output Port to Input Ports ofSingle Converter Module

The previous simulations imposed dc power transfer from inputs tooutput. Additional simulations are now performed to demonstrate thedc/dc converter topology in FIG. 1 is capable of bidirectional energyexchange, i.e. power transfer from inputs to output and vice versa.Moreover, power sharing between input ports is also demonstrated viasimulation.

Two additional simulations are performed for FIG. 1 to illustrate dcpower transfer from output port to input ports. Specifically, these twosimulated scenarios show that an energy source located at the outputtransfers 900 W to the inputs, and power sharing among the input portscan be controlled. Power sharing in this context implies the total powertransfer can be arbitrarily split between input ports.

FIG. 10 presents the simulated waveforms for power transfer from outputto inputs where the power is shared equally among the two input ports.FIG. 11 presents the simulated waveforms for power transfer from outputto inputs where the power is shared unequally among the two input ports,as assigned by the user.

Simulation Results: Realize Full Output Voltage Range for CascadedConverter Structure of FIG. 5

Simulations are provided to demonstrate the achievable output voltagerange for Zheng et al. in relation to the proposed topology in FIG. 5.The simulation models are implemented in PLECS and correspond to: 1)four series-cascaded two-quadrant dc/dc buck converter cells with singleL-C output filter (Zheng et al.), and 2) a four-input single-outputcascaded dc/dc converter structure of FIG. 5 (i.e. k=3). Each converterstructure has four batteries connected at the input terminals, whereeach battery has a nominal potential of 10 V. Thus, the possible rangeof output voltages is 0 to 40 V. It should be reiterated the former hasone buck converter cell dedicated to switch-mode operation while thelatter employs one converter module (comprising two buck convertercells) for possible switch-mode operation. The remaining converter cellsin each topology operate exclusively in low-frequency voltage stackingmode. As stated previously, it is assumed that duty cycle commands lessthan 0.1 or greater than 0.9 are unachievable.

FIG. 12 shows both the filtered and unfiltered average output voltage(i.e. “stack voltage”) corresponding to prior art, for all possibleoutput voltage references. Here one converter cell operates as a switchmode converter at any given time, while the remaining three cells areinserted as necessary to build up to the desired stack voltage. Observethe three regions in the lower plot of FIG. 12 that exhibit a “flat”voltage profile. This output response is a direct result of the dutycycle limitation of the switch-mode operating cell as describedpreviously. This simulation result clearly shows that prior art isincapable of achieving a continuous output voltage profile. Theconverter cannot provide certain output voltages as shown, and thereforeits operating range must be restricted to avoid these undesirableoperating regions. Alternatively a rapid sorting of the cell voltagescould address the problem, however, the cells pre-designated forlow-frequency operation could no longer be considered as operatingexclusively as such.

FIG. 13 shows both the filtered and unfiltered average output voltage(i.e. “stack voltage”) corresponding to the four-input single-outputstructure of FIG. 5, for all possible output voltage references.Individual cells within the high/low-frequency cell stack operate asswitch-mode converters when needed, and cells within the low-frequencycell stack operate exclusively in voltage stacking mode. The lower plotin FIG. 13 clearly demonstrates the topology of FIG. 5 is capable ofachieving a continuous output voltage profile. Thus, in comparison toprior art, the proposed topology can achieve better utilization ofavailable cell voltage in achieving the full output voltage range.

FIG. 14 shows gating signals for the individual converter cells of FIG.13. FIG. 15 shows a finer resolution time scale for a chosen segment ofsimulated waveforms in FIG. 14, to show contrast between high-frequencyand low-frequency switching times.

Simulation Results: Charge Balancing of Cells for Cascaded ConverterStructure of FIG. 5

Simulation results are provided to demonstrate the capability for cellbalancing in FIG. 5 based on the operation strategy depicted in FIG. 7.These PLECS simulations utilize the same case study system as utilizedin the previous simulation section. That is, a four-input single-outputrealization of FIG. 5 is modeled in PLECS with four integratedbatteries, where each battery has a nominal potential of 10 V.

FIG. 16 plots the four battery voltages as a fixed amount of dc power istransferred to the converter output terminals. Observe thestate-of-charge of all four batteries is depleted at the same average(i.e. long-term) rate. This charge balancing between cells is achievedby utilizing the operational strategy conceptualized in FIG. 7. Althoughimplemented as a case study example for FIG. 5, this same functionalitycan be implemented for FIG. 4 and FIG. 6 (or any variants thereof).Recall cell balancing can be achieved across all possible outputvoltages, which is not possible using prior art.

FIG. 17 shows gating signals for the individual converter cells of FIG.16. FIG. 18 shows a finer resolution time scale for a chosen segment ofsimulated waveforms in FIG. 17, to show contrast between high-frequencyand low-frequency switching times.

Variations

Whereas specific embodiments of the invention have been discussed,variations are possible. For example, FIG. 2 shows one possibleimplementation of the two pairs of complimentary switches using MOSFETsand Diodes, however, the switches can be implemented using a number ofdifferent switching devices and technologies, and, similarly, energystorage components (i.e. inductors and capacitors) can also beimplemented with equivalents. For example, the placement of capacitorsC_(3a) and C_(3b) in FIG. 19 can be employed as an alternate outputcapacitor configuration. All of these variations should be considered asfunctionally similar. It is also possible to realize unidirectionalvariants of the presented bidirectional topologies, by suitableimplementation of the switching devices.

Furthermore, FIG. 4 through FIG. 6 are examples of cascaded topologiesderived based on the converter module of FIG. 1 and series-cascaded buckconverter cells. As there are many other possible functionally similarrealizations of cascaded converter structures using the basic buildingblock in FIG. 1, the illustrated embodiments should not be considered aslimiting. Furthermore, the two-quadrant buck converter cell isexplicitly employed in all topologies, however, this is not essential.Other types of dc/dc converter cells may be employed, for example, buckor boost converter cells.

Whereas specific operating conditions and parameters are disclosed aspart of the simulations and others, persons of ordinary skill willunderstand that these are included for illustration, only, and are notintended to be limiting.

Another possible implementation of the topology can be observed in FIG.23, where the three-port structure has been changed to a two port one.This change implies differences on the operation of the converter, asgiven the input port connection it will be able to either step up ordown the output voltage, depending on the selected value for the dutycycle commands. The aforementioned structure does not alter thebidirectional capability of the topology, being able to transfer in bothdirections between the ports 1 and 2. In this case, the implementationof unidirectional variants it is also possible, by employing thesuitable switching devices.

Single-Input Single-Output Converter Module: Theory of Operation

As stated earlier, the topology provides the flexibility of beingoperated in different ways. Following the same assumptions from theearlier operational principle, the converter generates four switchingstates, which are illustrated in FIG. 24A, FIG. 24B, FIG. 24C, and FIG.24D. In order to determine the converter input/output relation, avolt-second balance analysis is performed once again. Given the factthat a single source is being used, this mode of operation wouldtypically employ balanced duty cycles with D₁≈D₂ such that both cellsprocess similar powers most of the time. While this is not necessary itsimplifies analysis and explanation.

Consider the converter is operating in steady state, with D₁=D₂=D. Therise and fall of inductor current during its charging and dischargingprocesses must be equal over one switching period, consequently, thefollowing voltage relationship can be derived when equating the averageinductor voltage to zero using IVSB:

$\begin{matrix}{V_{3} = {V_{4}\frac{D}{1 - D}}} & (8)\end{matrix}$Regulation of the Converter Using the Sum-Difference Domain

While the general input/output voltage ratio of (8) is sufficient forunderstanding the basic steady state behaviour of the converter fortypical use cases, for regulation of the converter requires a completemodel of the system dynamics is required, accounting for unequal D₁ andD₂. The converter has 3 dynamic state variables, one of which dependsexplicitly on the difference in duty cycles (D₁-D₂). Dynamics of theconverter are therefore most easily be examined through study of the sumand difference of the input capacitors' voltages; hence the followingvariables are introduced for control design:V _(Σ) =V ₁ +V ₂  (9)V _(Δ) =V ₁ −V ₂  (10)D _(Σ) =D ₁ +D ₂  (11)D _(Δ) =D ₁ −D ₂  (12)

Using FIG. 23 as reference and assuming that C₁=C₂=C_(d) andC_(a)=C_(b)=C_(a) and that V₃ is a known quantity, the dynamic equationsrelevant to the control of the converter can then be rewritten in termsof these quantities as follows:

$\begin{matrix}{{{L\frac{{di}_{L}}{dt}} + {R_{L}i_{L}}} = {\frac{V_{\sum}D_{\sum}}{2} + \frac{V_{\Delta}D_{\Delta}}{2} - V_{3}}} & (13) \\{{C_{d}\frac{{dV}_{\Sigma}}{dt}} = {{- I_{4}} - I_{3} + {i_{L}\left( {1 - D_{\Sigma}} \right)}}} & (14) \\{{\left( {C_{d} + C_{o}} \right)\frac{{dV}_{\Delta}}{dt}} = {{- D_{\Delta}}i_{L}}} & (15)\end{matrix}$

Notice from FIG. 23 that V₄=V₁+V₂−V₃ hence regulation of V₄ is achievedthrough control of V_(Σ) and V_(Δ) (which are convenient proxies for V₁and V₂). In cases where V4 is known and V3 is to be regulated, theequations are readily reformulated with help of the constraint equationV₄=V₁+V₂−V₃. In this case it is regulation of V₃ that is achievedthrough control of V_(Σ) and V_(Δ) (which are convenient proxies for V₁and V₂).

This above model leads to the control scheme presented in FIG. 25. Fromthe figure, it is possible to see that the difference voltage V_(Δ) isregulated by one control loop, most commonly it would tasked withmaintaining zero difference voltage, though a non-zero difference may berequested. The sum voltage V_(Σ) is regulated using a cascade controlstructure, that uses the inductor current i_(L) as an intermediatevariable. Once the sum and difference duty cycles have been obtained, D₁and D₂ are reconstructed and given to the pulse-width modulator. Pleasenote that there is a dependency of i_(L) on V_(Δ) and if this is notaddressed properly, it could lead to instability of the controller.Numerous methods exist in the literature to address this issue. A simpleapproach to reduce the coupling between the mentioned quantities is toselect a V_(Δ) control loop bandwidth that is significantly smaller thanthat of the current regulator. This will ensure that the influence ofV_(Δ) on the dynamics of i_(L) remains small.

Depending on the nature of the input source employed in the converter,the reference signals and the implementation of the regulator willdiffer slightly. For example, if V₃ and V₄ are both assigned by externalpower networks then V_(Σ) regulator is not needed at all. Instead theinductor current control loop will simply assign the amount of powerflow between V₃ and V₄, as a function of its set point.

Simulation Results: Example Case Study Performance for the ProposedSingle-Input Single-Output Converter Module with Unidirectional PowerFlow

In order to illustrate its operational principle and not limiting theapplication of the proposed control scheme, consider the case when theconverter has a solar photovoltaic array connected to its input, asshown in FIG. 25. In this case, V₃ must adjust be become equal to themaximum power point voltage of the solar array, and V₄ is assumedconstant. V_(Σ) is therefore regulated to be the difference between theoutput voltage and the maximum power point voltage of the array, whileV_(Δ) is set to zero to keep the input voltages balanced and operate ininterleaved mode.

The validation of the proposed single-input single output variation andits sum-difference control scheme is performed in Matlab/Simulink®,using the PLECS® toolbox. The unidirectional system is rated for 32 kWand the model of the PV arrays simulated is based on the Sharp/NUU235F1module, which has a rated power output of 235 W and 30 V under nominalconditions of temperature and solar irradiation. Considering this, eacharray comprises 34 series connected modules to reach the desired inputvoltage, and then these arrays are paralleled in order to meet the powerrequirements, in this case the array is comprised by 4 paralleledstrings. The remaining system parameters used in the simulation arepresented in Table 4

TABLE 4 Simulation Parameters for the Single-Input Single-Output StudyCase. Parameter Symbol Value dc-bus voltage V₃ 800 V Rated power P_(r)32 kW Input filter capacitance C_(d) 60 μF Output filter capacitanceC_(o) 10 μF Interleaved reactor inductance L 214.5 μH Interleavedreactor resistance R_(L) 1.8 m Ω Switching frequency f_(s) 20 kHz MPPTsampling time T_(m) 0.2 s PV string voltage V_(pv) 1020 V No. of stringsconnected in parallel N_(P) 4 No. of series connected PV modules perstring N 34 Open-circuit voltage of module V_(oc) 37 V Maximum powervoltage of module V_(pm) 30 V Short-circuit current of module I_(sc) 8.6A Maximum power of module I_(pm) 7.84 A

In order to test the dynamic performance of the system and also the MPPTcapability of the converter, the following scenario is imposed: thesystem starts with both of the arrays under standard test conditions,i.e., with a solar irradiance of 1 kW/m² and a temperature of 25° C.Then at t=0.35 s, the irradiance of the arrays is reduced to 0.6 pu.

Given the fact that this approach does not have the possibility ofasymmetrical generation, the dynamic scenario is changed to a simplereduction in the irradiance of the PV array, to illustrate the changesin the inductor current during lower power scenarios.

The results obtained for the study case are presented in FIG. 27. Fromthese results it is possible to appreciate some interesting differencesin terms of the regular operation of the converter. The converter startsgenerating its rated power and drops to 0.6 pu after the disturbance inthe irradiance, as presented in FIG. 27, (a). However, given that thesame power is always processed by both cells, the duty cycles do notdrift from each other after the disturbance takes place. This means thatthe converter remains operating in the interleaved mode regardless ofthe irradiance conditions. This situation is confirmed in FIG. 27, (b),where the duty cycles virtually exhibit no differences.

The previous statements are confirmed with the dynamic response of theinput voltages, shown in FIG. 27, (c) and FIG. 27, (d). In the mentionedfigures, V₁ and V₂ are maintained balanced for any scenario, with theexception of a brief transient due to the limited response of the MPPTalgorithm to the sudden change in irradiance. Consequently, given thefeatures of the proposed topology, if the input capacitors' voltageswere not modified throughout the test, the interleaved capacitorvoltages V_(a) and V_(b) also exhibit lower differences between them, ascan be seen in FIG. 27, (e).

The biggest drawback of the buck-boost operating mode is an increase inthe current handled by the inductor. The alternative connection of thePV arrays leads to the inductor handling the PV generated current inaddition to the output current. As presented in FIG. 27, (f) the averagecurrent flowing through the inductor has been scaled by a factor of

$\frac{1}{1 - d},$which suggests that the efficiency of the converter may be reduced incomparison to the cascaded buck operation.Simulation Results: Example Case Study Performance for the ProposedSingle-Input Single-Output Converter Module with Bidirectional PowerFlow

The single-input single-output variation of the proposed has the abilityof handling power in both directions, i.e., from port 1 to port 2 orvice versa. To validate the ability to reverse the current flow, theouter voltage controller in FIG. 25 has been eliminated, the currentreference is provided directly. The system will be driven fromexchanging power from the terminals connected to V₄, i.e., the input dcsource, toward the dc bus with voltage V₄, and then inverse this powerexchange. The power exchanges will be performed at rated value, to coverthe bigger power reversal possible in the system. The obtained resultsare presented in FIG. 30. It can be seen how the system reverses itspower flow with a smooth transition while the controlled variables arenot affected dramatically. FIG. 28, (a) exhibits the power being fed tothe dc bus, and has a positive value before t=0.15 s and then it startsdraining this rated value. As stated in the previous case study, theadvantages of the single sourced topology is that the asymmetriesbetween the upper and negative cells are marginal.

This is confirmed in FIG. 28, (b) which presents the duty cycles andbasically show no differences, even during the transients. The balancedoperation of the cells leads to an even distribution of the dc voltages,as presented in FIG. 28, (c) and FIG. 28, (d). The balanced voltageslead to the interleaved operation of the converter, allowing to maintainthe multiplicative effect of the switching strategy throughout theentire operation range. The smooth transition achieved by the converteris confirmed in FIG. 28, (e) where the evolution of the inductor currenti_(L) is exhibited. This figure also confirms the ability of thetopology to handle currents in both directions, allowing to charge ordischarge the dc load connected to its inputs terminals. This enablesthe use of the topology in bidirectional applications, such asinterfacing energy storage systems or fast charging electric vehiclesbattery packs.

Other Variations

The converter topology fundamentally displays 4 possible “ports”, namelyV₁, V₂, V₃ and V₄ and its structure imposes also one constraint:V₁+V₂+V₃=V₄. The foregoing discussion has focussed on the most commonchoices of ports that might be selected as converter “input/output”, butthis should not be limiting. For example, V₂ and V₄ could be chosen as“input/output” ports, leaving the voltages V₃ and V₄ as internalconverter variables. While requires a change in regulation,bi-directional power flow between these newly chose ports is offered bythe topology.

Description of FIG. 23

FIG. 23 shows a single-input single-output application of the convertermodule, using the functionally similar output capacitor configuration.In FIG. 23, the circuit has a first port and a second port. FIG. 23shows a circuit having four capacitors, C1, C2, Ca, and Cb. The circuithas an inductor L, along with switches S1 a, S1 b, S2 a, and S2 b. Cahas (i) a positive terminal coupled to a positive node of a firstswitching cell that is electronically coupled to the positive terminalof C1, and (ii) a negative terminal coupled to a positive node of asecond switching cell of the two switching cells that is electronicallycoupled to the positive terminal of C2. Cb has (i) a positive terminalcoupled to a negative node of the first switching cell of the twoswitching cells that is electronically coupled to the negative terminalof C1, and (ii) a negative terminal coupled to a negative node of thesecond switching cell of the two switching cells that is electronicallycoupled to the negative terminal of C2.

The invention claimed is:
 1. A system for interconnecting a plurality ofenergy storage devices to a DC bus, the system comprising: at least oneinterconnection circuit comprising: two switching cells including afirst switching cell and a second switching cell, each switching cellincluding a pair of terminals and a switching node; a first capacitorhaving a (i) positive terminal coupled to a positive node of a firstswitching cell of the two switching cells, and a (ii) negative terminalcoupled to a negative node of the first switching cell of the twoswitching cells; and a second capacitor having a (i) positive terminalcoupled to a positive node of the second switching cell of the twoswitching cells, and a (ii) negative terminal coupled to a negative nodeof the second switching cell of the two switching cells; and anadditional capacitor or capacitor network, providing a capacitancebetween the first switching cell and the second switching cell; a filterinductor connected between the two switching cells; wherein a DC load orsource is connected between the positive terminal of the secondswitching cell and the negative terminal of the first switching cell;wherein the DC bus is connected between the negative terminal of thesecond switching cell and the positive terminal of the first switchingcell; a switching cell controller configured to: regulate the differencein voltage between a first capacitor voltage of the first capacitor anda second capacitor voltage of the second capacitor; and regulate acurrent exchange between the DC bus and the DC load or source.
 2. Thesystem of claim 1, wherein a first switching cell of the two switchingcells operates at a duty cycle D₁, and wherein a second switching cellof the two switching cells operates at a duty cycle D₂.
 3. The system ofclaim 2, wherein each switching node for each switching cell comprisestwo switches, which operate alternatively such that the duty cycles D₁and D₂ control a percentage time that a corresponding first switch ofthe two switches for the corresponding switching node is conducting, andthe corresponding second switch of the two switches for thecorresponding switching node is not conducting.
 4. The system of claim2, wherein D₁ is approximately equal to D₂ during steady-stateoperation.
 5. The system of claim 2, wherein a difference in duty cycles(D₁-D₂) is used for regulation of the at least one interconnectioncircuit.
 6. The system of claim 5, wherein the at least oneinterconnection circuit is configured to operate based at least on atleast 3 dynamic state variables, (a) the difference in duty cycles(D₁-D₂), (b) V_(Σ)=V₁+V₂, and (c) V_(Δ)=V₁-V₂, wherein V₁ is a voltageacross an input capacitor of one of the two switching cells, and V₂ is avoltage across an input capacitor of the other of the two switchingcells.
 7. The system of claim 6, wherein V₃ is the target voltage at theDC bus, and V₄ is a voltage difference between the two switching cells,and wherein V₃ or V₄ are regulated by control of V_(Σ) and V_(Δ).
 8. Thesystem of claim 7, wherein V_(Σ) is regulated using a cascade controlstructure that uses an inductor current i_(L) of the filter inductor asan intermediate variable.
 9. The system of claim 7, wherein V_(Δ) isregulated using a control loop.
 10. The system of claim 6, wherein afterthe V_(Σ) and the difference in duty cycles (D₁-D₂) have been obtained,D₁ and D₂ are reconstructed and provided to the switching cellcontroller.
 11. A method for interconnecting a plurality of energystorage devices to a DC bus using at least one interconnection circuitincluding two switching cells including a first switching cell and asecond switching cell, each switching cell including a pair of terminalsand a switching node, the at least one interconnection circuit includinga first capacitor having a (i) positive terminal coupled to a positivenode of a first switching cell of the two switching cells, and a (ii)negative terminal coupled to a negative node of the first switching cellof the two switching cells; and a second capacitor having a (i) positiveterminal coupled to a positive node of the second switching cell of thetwo switching cells, and a (ii) negative terminal coupled to a negativenode of the second switching cell of the two switching cells; and anadditional capacitor or capacitor network, providing a capacitancebetween the first switching cell and the second switching cell, and afilter inductor connected between the two switching cells; wherein a DCload or source is connected between the positive terminal of the secondswitching cell and the negative terminal of the first switching cell;wherein the DC bus is connected between the negative terminal of thesecond switching cell and the positive terminal of the first switchingcell, the method comprising: regulating a difference in voltage betweena first capacitor voltage of the first capacitor and a second capacitorvoltage of the second capacitor; and regulating a current exchangebetween the DC bus and the DC load or source.
 12. The method of claim11, wherein a first switching cell of the two switching cells operatesat a duty cycle D₁, and wherein a second switching cell of the twoswitching cells operates at a duty cycle D₂.
 13. The method of claim 12,wherein each switching node for each switching cell comprises twoswitches, which operate alternatively such that the duty cycles D₁ andD₂ control a percentage time that a corresponding first switch of thetwo switches for the corresponding switching node is conducting, and thecorresponding second switch of the two switches for the correspondingswitching node is not conducting.
 14. The method of claim 13, wherein adifference in duty cycles (D₁-D₂) is used for regulation of the at leastone interconnection circuit.
 15. The method of claim 13, wherein the atleast one interconnection circuit is configured to operate based atleast on at least 3 dynamic state variables, (a) the difference in dutycycles (D₁-D₂), (b) V_(Σ)=V₁+V₂, and (c) V_(Δ)=V₁-V₂, wherein V₁ is avoltage across an input capacitor of one of the two switching cells, andV₂ is a voltage across an input capacitor of the other of the twoswitching cells.
 16. The method of claim 15, wherein V₃ is the targetvoltage at the DC bus, and V₄ is a voltage difference between the twoswitching cells, and wherein V₃ or V₄ are regulated by control of V_(Σ)and V_(Δ).
 17. The method of claim 15, wherein V_(Σ) is regulated usinga cascade control structure that uses an inductor current i_(L) of thefilter inductor as an intermediate variable.
 18. The method of claim 15,wherein after V_(Σ) and the difference in duty cycles (D₁-D₂) have beenobtained, D₁ and D₂ are reconstructed and provided to the switching cellcontroller.
 19. A non-transitory computer readable medium storingmachine interpretable instructions, which when executed by a processor,cause the processor to execute a method for interconnecting a pluralityof energy storage devices to a DC bus using at least one interconnectioncircuit including two switching cells including a first switching celland a second switching cell, each switching cell including a pair ofterminals and a switching node, the at least one interconnection circuitincluding first capacitor having a (i) positive terminal coupled to apositive node of a first switching cell of the two switching cells, anda (ii) negative terminal coupled to a negative node of the firstswitching cell of the two switching cells; and a second capacitor havinga (i) positive terminal coupled to a positive node of the secondswitching cell of the two switching cells, and a (ii) negative terminalcoupled to a negative node of the second switching cell of the twoswitching cells; and an additional capacitor or capacitor network,providing a capacitance between the first switching cell and the secondswitching cell; wherein a DC load or source is connected between thepositive terminal of the second switching cell and the negative terminalof the first switching cell; wherein the DC bus is connected between thenegative terminal of the second switching cell and the positive terminalof the first switching cell, the method comprising: regulating adifference in voltage between a first capacitor voltage of the firstcapacitor and a second capacitor voltage of the second capacitor; andregulating a current exchange between the DC bus and the DC load orsource.
 20. The non-transitory computer readable medium of claim 19,wherein a first switching cell of the two switching cells operates at aduty cycle D₁; wherein a second switching cell of the two switchingcells operates at a duty cycle D₂; wherein each switching node for eachswitching cell comprises two switches, which operate alternatively suchthat the duty cycles D₁ and D₂ control a percentage time that acorresponding first switch of the two switches for the correspondingswitching node is conducting, and the corresponding second switch of thetwo switches for the corresponding switching node is not conducting;wherein a difference in duty cycles (D₁-D₂) is used for regulation ofthe at least one interconnection circuit; and wherein the at least oneinterconnection circuit is configured to operate based at least on atleast 3 dynamic state variables, (a) the difference in duty cycles(D₁-D₂), (b) V_(Σ)=V₁+V₂, and (c) V_(Δ)=V₁-V₂, wherein V₁ is a voltageacross an input capacitor of one of the two switching cells, and V₂ is avoltage across an input capacitor of the other of the two switchingcells.